Artificial intervertebral disc

ABSTRACT

An artificial intervertebral disc including housing members including spaced inner surfaces facing each other and oppositely facing outer surfaces for engaging spaced apart intervertebral surfaces; self-adjusting bearing mechanisms operatively disposed between the inner surfaces for moving relative to the housing members to adjust and compensate for vertebral disc motion; and positioning ring for controlling motion and position of the bearing mechanisms and for absorption of compressive loads. An artificial intervertebral disc including housing members having an oval recess on the inner surfaces; oval bearing mechanisms operatively disposed within the oval recess between the inner surfaces for moving relative to the housing members to adjust and compensate for vertebral disc motion; and oval positioning ring. A spring member for an artificial intervertebral disc including a substantially annular body having an axially extended bore therethrough defining a passageway.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a Continuation-In-Part application of U.S.patent application Ser. No. 10/653,540, filed Sep. 2, 2003, which is aContinuation-In-Part application of U.S. patent application Ser. No.10/430,861, filed May 6, 2003, which are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to a spinal implant assembly forimplantation into the intervertebral space between adjacent vertebralbones to provide stabilization and continued postoperative flexibilityand proper anatomical motion. More specifically, the present inventionrelates to an artificial intervertebral disc, sometimes referred to asan intervertebral spacer device, for functioning as a load sharing andbearing device for replacement of the damaged, decayed, or otherwisenonfunctioning intervertebral disc.

2. Background of the Invention

The spine is a complex structure consisting of multiple flexible levels.Each level consists of a system of joints defined by adjacent vertebralbones. The system of joints includes intervertebral discs, which are atwo-part structure. The disc consists of a nucleus and an annulus. Thesystem allows motion while the facet joints add posterior stabilizationto the spinal column. The disc allows motion and cushioning to thejoint.

The complex system of the joint is subjected to varying loads andproblems over time, including disc degeneration due to a variety ofreasons. Disc degeneration can be attributed to aging, damage due toexcessive loading, trauma, and other anatomical issues. Facet joints ofthe structure can be compromised due to the same reasons, as well as dueto arthritic changes. Severe joint degeneration and failure can oftencause sufficient pain to require surgical intervention.

The current standard method of treatment for severe pain caused by spinejoint problems is fusion at the damaged level of the spine. Thetreatment, when successful, fuses the damaged section into a single massof bone. The fusion of the joint eliminates motion of the joint, therebyreducing or eliminating pain at that level. Success rates for painelimination are very high for this method of treatment. However, sincethe entire spine works as a system, fusion results in complications.

Elimination of motion at the spine alters the biomechanics of the spineat every other level. If one level is fused, then loads are absorbed byone less disc into a system not designed for such change. Thus, theremaining discs must redistribute loads, each disc absorbing a greaterload. In addition, the spine flexes to absorb loads. A fusion alters themeans by which the spine flexes, which also increases the loads on theremaining healthy discs. In turn, it is well understood that acomplication of fusion is that additional fusions may be required in thefuture as the other discs deteriorate due to the altered biomechanics ofthe spine. In other words, short-term pain relief is exchanged forlong-term alterations of the spine, which, in turn, usually requirefurther surgery.

There are numerous prior art patents addressing the issue of discreplacement. The U.S. Pat. Nos. 6,443,987 B1 and 6,001,130, both toBryan, disclose polymer composite structures for cushioningintervertebral loads. The U.S. Pat. Nos. 5,258,031 to Salib, et al. and5,314,477 to Marnay disclose ball and socket type implants addressingthe issue of intervertebral mobility. These patents are exemplary of afirst approach using an elastomer as a motion and dampening structureand a second approach utilizing a ball and socket joint to create amoving pivot joint. There are many variations on these concepts, whichinclude mechanical springs and more complex structural mechanisms. Asignificant portion of the prior art addresses the issues ofintervertebral motion but do not address anatomical loadingconsiderations.

The current state of prior art artificial intervertebral discs areassociated with various problems. For example, a number of implantsconstructed from polymers are of insufficient strength to workeffectively in the higher loading areas, such as the lumbar spine. Suchpolymers often take compressive sets so that the original height of theimplant decreases over time. A surgeon must either compensate for thecompression by initially using a larger polymer prosthesis and estimatecompression or use the appropriately sized polymer prosthesis and latersurgically replace the same once the irreversible compression of theprosthesis is unacceptable.

Implants constructed with ball and socket joints severely restrict oreliminate shock cushioning effect of a normal disc. This implant canprovide motion, but biomechanically, the ball and socket jointnegatively affects other healthy discs of the spine. The result can belong-term problems at other levels of the spine, as seen with thecurrent treatment of fusion.

Other implants, not discussed above, utilize bearing surfaces usuallyhaving polyethylene bearing against metal interfaces. Polyethylene as abearing surface is problematic in large joint replacement due to thewear properties of the material. Since artificial discs are intended tobe implanted over long periods of time, such wear can be highly damagingto surrounding tissue and bone.

In view of the above, it is desirable to provide a solution tointervertebral disc replacement that restores motion to the damagednatural disc area while allowing for motion as well as cushioning anddampening, similar to the naturally occurring disc. In addition, it ispreferable to allow such motion, cushioning, and dampening whilepreventing a polymer or elastomeric material from experiencing therelatively high compressive loads seen in the spine. It is alsopreferable to allow a bearing surface to share the spinal loads with thepolymer and elastomeric material. Finally, it is preferable to controlchanges to the artificial motion intraoperatively to adjust foranatomical conditions.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an artificialintervertebral disc including housing members having spaced innersurfaces facing each other and oppositely facing outer surfaces forengaging spaced apart intervertebral surfaces; self-adjusting bearingmechanisms operatively disposed between the inner surfaces for movingrelative to the housing members to adjust and compensate for vertebraldisc motion; and positioning ring for controlling motion and position ofthe bearing mechanisms and for absorption of compressive loads. Alsoprovided is an artificial intervertebral disc including housing membershaving spaced inner surfaces facing each other and oppositely facingouter surfaces for engaging spaced apart intervertebral surfaces,wherein the inner surfaces include an oval recess thereon; oval bearingmechanisms operatively disposed within the oval recess between the innersurfaces for moving relative to the housing members to adjust andcompensate for vertebral disc motion; and oval positioning ringoperatively engaged with the oval recess and oval bearing mechanisms forcontrolling motion and position of the bearing mechanisms and forabsorption of compressive loads between the bearing mechanisms and thehousing members. The present invention further provides a spring memberfor an artificial intervertebral disc including a substantially annularbody having an axially extended bore therethrough defining a passageway.

DESCRIPTION OF DRAWINGS

Other advantages of the present invention can be readily appreciated asthe same becomes better understood by reference to the followingdetailed description when considered in connection with the accompanyingdrawings wherein:

FIG. 1 is a side perspective view of a preferred embodiment of thepresent invention;

FIG. 2 is a side exploded view of the embodiment shown in FIG. 1;

FIG. 3 is a side perspective view of a second embodiment of the presentinvention;

FIG. 4 is a perspective view of a lower disc constructed in accordancewith the present invention;

FIG. 5 is a side view of an upper disc constructed in accordance withthe present invention;

FIG. 6 is a top perspective view of an upper housing member made inaccordance with the present invention;

FIG. 7 is a top plan view of a lower housing member made in accordancewith the present invention;

FIG. 8 is a side perspective view of a third embodiment of the presentinvention;

FIG. 9 is a perspective view of the present invention with the tophousing member removed;

FIG. 10 is a perspective view of an alternative pad configuration of thepresent invention;

FIG. 11 is a perspective view of a further alternative embodiment of thepad member;

FIG. 12 is a further alternative embodiment of the present invention;

FIG. 13 is an exploded side perspective view of the embodiment shown inFIG. 12;

FIG. 14 shows an alternative embodiment of the housing members of thepresent invention;

FIG. 15 shows a further alternative embodiment of the housing members ofthe present invention;

FIG. 16 is an exploded view of a further embodiment of the presentinvention demonstrating a bayonet type locking of a disc member to ahousing member;

FIG. 17 is a perspective view of the disc member utilizing the bayonetlocking mechanism to lock the disc member within a housing member;

FIG. 18 is an exploded view of a disc member and housing member showinga further embodiment of a locking mechanism for locking the disc memberwithin the housing member;

FIG. 19 is a perspective view showing the disc member locked within thehousing member,

FIG. 20 is a perspective view of the a further embodiment of the housingmember;

FIG. 21 is a cross sectional view taken along line 21—21 in FIG. 20;

FIG. 22 is a perspective view of a load sharing pad member includingflanges for locking engagement in the recesses of the housing membershown in FIGS. 20 and 21;

FIG. 23 shows a further embodiment of a locking mechanism made inaccordance with the present invention;

FIG. 24 is a top view of the mobile bearing of the present invention;

FIG. 25 is a top view of the artificial disc including a mobile bearingwith no load sharing pads;

FIG. 26 is a top view of the multidirectional mobile bearing of thepresent invention;

FIGS. 27A and B are side views of the mobile bearing of the presentinvention;

FIG. 28 is a side perspective view of the mobile bearing of the presentinvention resting in a seat;

FIG. 29 is a top perspective view of the seat and bearing combination ina housing having recesses for load sharing pads;

FIG. 30 is a side perspective view of a third embodiment of the presentinvention;

FIG. 31 is a perspective view of the base plate of a third embodiment ofthe present invention;

FIG. 32 is a side view of a third embodiment of the lower housing of thepresent invention;

FIG. 33 is a perspective view of the third embodiment of the presentinvention wherein a spherical surface is incorporated on the bearing;

FIG. 34 is a perspective view of the third embodiment of the presentinvention wherein a spherical surface is incorporated on the bearing;

FIG. 35 is a side view of the third embodiment of the present invention;

FIG. 36 is a side view of the third embodiment of the present invention;

FIG. 37 is a side perspective view of an alternative embodiment of thepresent invention;

FIG. 38 is a perspective view of the base plate of the third embodimentof the present invention wherein the bearing is either convex orconcave;

FIG. 39 is a perspective view of the base plate of the third embodimentof the present invention wherein the bearing is either convex orconcave;

FIG. 40 is a top perspective view of the bumpers of the presentinvention;

FIG. 41 is a perspective view of an embodiment of the housing members ofthe present invention, wherein the housing members include apertures forbone screws and a positioning ring;

FIG. 42 is a perspective view of an embodiment of the housing members ofthe present invention, wherein a recess is shown for accommodating thepositioning ring and bearing discs;

FIG. 43 is a perspective view of an embodiment of the housing memberthat is oval-shaped;

FIG. 44 is a perspective view of an oval-shaped positioning ring;

FIG. 45 is a perspective view of the oval-shaped positioning ring,bearing disc, and housing member;

FIG. 46 is a side view of an upper housing member including a fixedbearing disc;

FIG. 47 is a cut away view of the disc of the present invention showingengagement of the bearing surfaces and engagement of the ovalpositioning ring, wherein the bearing disc is oval shaped and the recesson the housing member is oval-shaped;

FIG. 48 is a perspective view of the disc assembly of the presentinvention;

FIG. 49 illustrates the insertion of a trial into the disc space;

FIG. 50 illustrates a drill guide for use in drilling pilot holes at aguide plate locations;

FIG. 51 illustrates securing the guide plate with self-tapping guideplate screws;

FIG. 52 illustrates inserting reaming discs matching the trial numberinto the disc assembly;

FIG. 53 illustrates engagement of the trial with the disc assembly;

FIG. 54 illustrates removal of guide plate screws and guide plate;

FIG. 55 illustrates insertion of disc holder with holes in plate;

FIG. 56 illustrates insertion of screws into threaded holes to securedisc to the vertebral bodies; and

FIG. 57 illustrates attached disc assembly.

DETAILED DESCRIPTION OF THE INVENTION

An artificial intervertebral disc constructed in accordance with thepresent invention is generally shown at 10 in the Figures. Similarstructures of various embodiments are indicated by primed numerals inthe Figures. The invention is an artificial intervertebral disc,sometimes referred to by other terminology in the prior art such asintervertebral spacer device, or spinal disc for replacement of adamaged disc in the spine. The invention restores motion to the damagednatural disc that allows for motion as well as cushioning and dampening.As described below in more detail, the present invention also allowschanges to the artificial disc motion intraoperatively to adjust forspecific anatomical conditions.

Referring to the Figures, the disc 10 includes an upper housing membergenerally shown at 12 and a lower housing member generally shown at 14.The housing members 12, 14 include spaced inner surfaces 16 and 18facing each other and oppositely facing outer surfaces 20, 22 forengaging spaced apart vertebral surfaces. A pair of bearing surfaces 24,26 extend from each of the inner surfaces 16, 18 for engaging each otherwhile allowing for low friction and compression resistant movement ofthe housing members 12, 14 relative to each other while undercompression. As shown in the various Figures, the bearing surfaces areintegral with disc members 28, 30. The housing members 12, 14 can bemade from various materials including metals, such as titanium, as wellas ceramics, and plastics. Additionally, the housing members 12, 14 canbe coated with materials to reduce friction between the components ofthe disc 10, specifically between the housing members 12, 14 and bearingdisc members 28, 30. Coating materials include, but are not limited to,TiN (Titanium Nitride), diamond, diamond-like materials, syntheticcarbon-based materials, chromium-based materials, and any other similarcoating materials known to those of skill in the art. If integral withthe bearing surfaces 24, 26, the housing members 12, 14 can be made fromthe preferred material for the bearing discs 28, 30 as discussed above.Based on this teaching, various other configurations can be made bythose skilled in the art incorporating the present invention.

The bearing surfaces 24, 26 preferably form a mobile bearing 23 that iscapable of automatically adjusting the position of the bearing 23 withina housing 14 as needed. The mobile bearing 23 is shown in FIGS. 24through 29. The bearing 23 is preferably made of any material thatslides along the surface of the housing 14 in which it is placed, withminimal to no wear, on either the bearing 23 or the housing 14. Examplesof such materials include ceramic, metal, or other suitable materialsthat do not negatively react with the housing 14.

The bearing 23 of the present invention is disposed within a slot 35 ofa housing 14. The bearing 23 is able to freely move or float within theslot 35 in response to movement of the housing 14. The bearing 23 isdesigned to provide proper cushioning and support of the housing 14 asis required by the specific system in which the bearing 23 is placed.The bearing 23 can be used in any joint for providing proper support ofthe joint. For example, if the bearing 23 is used in an artificialintervertebral disc assembly, the bearing 23 provides cushioning so asto prevent the plates that are housing the disc from touching andwearing on one another. When the bearing 23 is utilized within the knee,the bearing also provides cushioning for the housing 14 during movementof the housing 14.

The bearing 23 disclosed herein can move freely under load conditionswhile maximizing the contact area of the upper and lower bearingsurfaces 20, 24. In other words, within the slot 35 that the bearing 23is disposed, the bearing 23 can move in any direction necessary toprovide the proper support for the housing 14. The bearing 23 is able tomove in this manner because the bearing 23 is a floating bearing, thusit is not attached or affixed to the housing 14 in which it is placed.Instead the bearing 23 “floats” within the housing 14, thus enabling thebearing 23 to be mobile and free to move in any direction necessary toprovide proper support.

The housing 14 limits the “floating” motion of the bearing 23. In otherwords the movement of the bearing 23 can be limited based upon the sizeof the housing 14 and more specifically the slot 35 in which the bearing23 is disposed. The slot 35 in which the bearing 23 is disposed dictatesthe range of movement of the bearing 23, i.e. movement can beconstrained such that the bearing 23 can only move from an anterior to aposterior position. More specifically, the slot includes side walls 37,which define the size and shape of the slot 35, and a seat 39 on whichthe bearing is disposed. The movement of the bearing 23 is restrictedbased upon the shape of the walls 35 of the slot 35 in which the bearing23 sits. For example, the slot 35 can be in the shape of a circle, anoval, or any other round-sided shape. The slot 35 must be shaped to haverounded sides so as to prevent the bearing 23 from lodging in a cornerof the slot 35. The slot 35 can be formed such that the seat 39 does nothave a uniform depth, such that there are peaks or angles within theslot 35, as shown in FIG. 27. The lack of uniformity restricts movementof the bearing 23 within the slot 35 because the bearing 23 wouldrequire additional force in order to slide in the direction of the peakor angle.

A removable insert 33, as shown in FIGS. 28 and 29, can also be disposedwithin the housing 14 for holding the bearing 23 in place. The insert 33includes an upper surface 29 for engaging the bearing surfaces 24, 26.The insert 33, can be made of any material that enables the bearing 23to functionally “float” across the insert 33 without excessive friction.The benefit of including the insert 33 in a housing 14 is that theinsert 33 can be made of a different material than that of the housing14. Accordingly, the housing 14 can be made from a first compositionthat is advantageous for the functionality of the housing and providesother strength characteristics while the insert 33 can be made from amore lubricious material to allow for more efficient friction-freemovement of the bearing 23 thereon.

The movement of the bearing 23 is restricted based upon the shape of theinsert 33 into which the bearing 23 is placed. The insert 33 includesside walls 41, which define the size and shape of the insert 33, and aninsert seat 29 on which the bearing is disposed. The movement of thebearing 23 is restricted based upon the shape of the walls 41 of theinsert 33 in which the bearing 23 sits. For example, the insert 33 canbe in the shape of a circle, an oval, or any other round-sided shape.The insert 33 must be shaped to have rounded sides so as to prevent thebearing 23 from lodging in a corner of the insert 33. The insert 33 canbe formed such that the insert seat 29 does not have a uniform depth,such that there are peaks or angles within the insert 33, as shown inFIG. 27. The lack of uniformity restricts movement of the bearing 23within the insert 33 because the bearing 23 would require additionalforce in order to slide in the direction of the peak or angle.

The housing 14 can also include load distributing dampening andcushioning pad recesses 32, 58. Load sharing pads 32, 34 generally shownat 31 and specifically indicated as pads 32 and 34 in FIGS. 1 and 2 aredisposed between the inner surfaces 16, 18 and about at least a portionof the bearing surfaces 24, 26 for sharing absorption of compressiveloads with the bearing surfaces 24, 26 while limiting relative movementof the housing members 12, 14. More specifically, under in vivo loadingconditions, the centralized bearing surfaces 24, 26 and the floatingbearing surfaces not only provide for three-dimensional movementrelatively between the housing members 12, 14, but also share with theload sharing pads 32, 34 the function of distributing compressive loadson the device 10 to provide a system for motion and effective loaddistribution. The centralized low friction and compression resistantbearing surfaces 24, 26 allow full motion in multiple planes of thespine while the load distributing damper and cushioning pads 32, 34simultaneously share the load. Critical is the function of the pads 32,34 sharing the load with the bearing surfaces 24, 26. Although the pads32, 34 can be compressible, the compression is limited by thenoncompressibility of the bearing surfaces 24, 26. Likewise, althoughthe bearing surfaces allow for motion in multiple planes, the pads 32,34 are fixedly secured to the housing members 12, 14, thereby allowingfor a degree of flexibility and therefore movement of the housingmembers 12, 14 relative to each other, yet limiting such movement. Intotal, each element, the bearing surfaces 24, 26, and pads 32, 34, allowfor movement, yet limit such movement, whether it is the slidingmovement of the bearing surfaces 24, 26 or the cushioning movementallowed by the pads 32, 34. Each element allows for relative movement,yet each element limits the movement of the other element of the system.

In view of the above, the system allows restoration of normal motionwhile maintaining load cushioning capabilities of a healthy disc. Thisis particularly apparent with motion of the spine. Any rotation of theupper and lower housing members 12, 14 causes the load distributingdampening and cushioning pads 32, 34 to absorb some of the load.

As shown in the various Figures, the bearing surfaces 24, 26 can includea concave surface portion on one of the upper or lower disc members 28,30, and a convex surface portion on the other. The concave surface isseated within the convex surface for sliding movement relative theretoeffectively resulting in relative pivoting motion of the housing members12, 14, which compresses at least a portion of the load sharing pads 32,34 while extending at least a portion of the oppositely disposed loadbearing pad 32, 34. Alternatively, either one of the top and bottom discmembers 28, 30 can have either of the convex or concave surfaces.

The disc members 28, 30 can be made from a composition that isnoncompressible. Such compositions can be selected from the groupincluding ceramics, plastics, and metal bearing materials, such ascobalt and chrome. Alternatively, the housing members 12, 14 can includeprojections wherein the disc members 28, 30 are effectively integralwith the housing members 12, 14. In this situation, the entire housing,including the projections having the bearing surfaces 24, 26 thereon,can be made from the noncompressible material, preferably a ceramic. Asstated above, alternative configurations can be made by those skilled inthe art once understanding the present invention.

The load sharing pads 32, 34 can be in various configurations shown inthe Figures, such as paired pads 32, 34 shown in FIGS. 1–3.Alternatively, the device 10 can include four oppositely disposed pads38, 40, 42, 44 as shown in FIG. 10. A further embodiment of theinvention is shown in FIG. 11, wherein a single pad 46 substantiallycovers the surface 18″″′ of the housing member 14′′″. The pads cancontour to the shape of the housing members such as shown in FIGS. 12,13, wherein the pad member 48 is an annular pad member disposed with aannular housing 12″″″, 14″″″. The selection of such housing members 12,14 and pad members 31 can be determined based on the location of theplacement of the device 10 as well as the spacing conditions between thevertebrae and load bearing necessities depending on the level of thespine being addressed. In other words, different shaped devices, such asthe round shaped housing members shown in FIG. 12 can be used forplacement between smaller discs, such as cervical spines whereas morerectangular shapes, such as the housing members shown in FIGS. 1–11 canbe used in between lumbar vertebrae.

The load sharing pads 31, in which ever shape they are configured, areelastic for allowing relative twisting movement between the housingmembers 12, 14 effecting relative three-dimensional movement between thehousing members 12, 14, while limiting the movement and preventingcontact between the housing members 12, 14 except for the contactbetween the bearing surfaces 24, 26. By elastic, it is meant that thepad members 31 are compressible and stretchable, yet provide aself-centering effect on the assembly with specific regard to thehousing members 12, 14, as well as the bearing surfaces 24, 26.Deflection or rotation of the forces created due to relative movement ofthe bearing surfaces 24, 26, and likewise the housing members 12, 14,forces the pads 31 to act in such a way to counter the force, thusallowing a unique self-centering capability to the assembly 10. While inan ideal situation, wherein the patient's facets are uncompromised andligamental balances are intact, this self-centering aspect may not becompletely necessary. In other words, the patient's anatomy may stillprovide stabilization and specifically, ligaments may provideself-centering. However, ligamental imbalance, and damaged facets wouldnormally make an artificial disc questionable, at best, with use of thecurrent technology that is available. In such cases, having the abilityto self-center and restrict motion (the pads 31 of the present inventionare elastic and thus restrict motion by stretching and returning torest), the possibility of extending indications to patients currentlyconsidered outside of the scope of artificial disc technology will behighly advantageous.

The pads 31 of the present invention provide further advantages to theinvention. A key advantage is the ability to adjust the pads 31 topatient and surgeon requirements. In such cases wherein range of motionneeds to be restricted due to compromised facets, a harder, less elasticpad can be inserted between the housing members 12, 14. Since this lesselastic pad would move and stretch less, the disc would be automaticallyrestricted in motion. This method of adjusting pads can be doneintraoperatively to compensate for surgical and patient conditions. Toone skilled in the art, one can fine-tune the assembly 10 to a patientand surgeon's needs with multiple pads of different properties ormaterials.

The pads 31 are made from a polymer or elastomer that allows deflectionunder load. Examples of such polymers and elastomers are silicone,polyurethane, and urethane composites. As discussed above with regard toflexibility or elasticity, the content and composition of the pads 31are adjustable. A highly dense material creates a very rigid disc, whilea very soft material creates a very free moving disc. The motion wouldbe restricted in all planes of the pad depending upon these factors.Rotation is also restricted, as well as flexion or movement of the disc.The amount of compression possible is restricted or allowed according tothe pads material properties. This is true of motion towards the back orside-to-side motion. Thus, the pads 31 are always in contact and alwaysshare the load, under any adjustment of relative positioning of thehousing members 12, 14. Since motion forces the pads to be in contact,the pads 31 automatically damper loads imposed by the artificial discconstruct 10.

With specific regard to the flexibility or elasticity of the polymer orelastomer composition of the pads 31, the pads can be selected from acomposition having a durometer from 20 to 98 on the Shore OO Scale.Alternatively, the pads 31 can be selected from a composition having adurometer from 10 to 100 on the Shore A Scale. A further alternative isfor the pads 31 to be selected from a composition having a durometerfrom 22 to 75 on the Shore D Scale. In any event, the pad members 31 canbe selected during the operation and procedure by the clinician to suita specific situation. Although the pad members 31 can be pre-insertedbetween the housing members 12, 14 prior to insertion of the device 10in situ, the various configurations of the present invention can allowfor in situ replacement of the pad members 31 so as to custom select theflexibility or elasticity of the members. In this manner, the padmembers 31 are custom designed for the individual environment of theintervertebral space into which the device is being disposed.

The disc members 28 and 30, and pads 31 can be contained or locked inposition in between the housing members 12, 14 by various means. Thedisc 28, 30 can be locked to the housing members 12, 14 by a press fittaper, retaining ring, or other means. The key aspect of such lockingmechanisms is to prevent the disc members 28, 30 from moving against theupper or lower housing members 12, 14 once installed in order to preventadditional wear.

FIGS. 1 and 2 show disc members 28, 30 disposed in recesses (only thelower recess 50 is shown in FIG. 2 in an exploded view) in each of theinner surfaces 16, 18 of the housing members 12, 14. FIGS. 6 and 7 showplan views of a second embodiment of the housing member 12′, 14′,wherein each recess 50′, 52 includes a ramped surface 54, 56 leadingfrom an outer edge to the inwardly tapered recess portion 50′, 52. Theramping 54, 56 allows access of the disc members 28,30 in between thehousing members 12′, 14′ after placement of the housing members 12′, 14′in the intervertebral space. This intraoperative access of the discmembers 28, 30 allows the surgeon to test different size disc membersunder load conditions to perfectly fit the disc members in place. Suchan advantage is not obtainable with any prior art device.

An alternative mechanical mechanism for locking the disc members withinthe housing members is shown in FIG. 16. The representative housingmember 12′″ includes recess 52′. The recess 52′ includes a substantiallyarcuate peripheral undergroove 70. The groove is defined by a lipportion 72 including at least one and preferably at least two openings74, 76. The disc member 28′″ includes bayonet style flanges 78, 80extended radially outwardly therefrom, the flanges 78, 80 being shapedso as to be received through recess 74, 76. In operation the disc member28′″ can be disposed within the recess 52′ such that the flanges 78, 80align with recesses 74, 76. Once the disc member 28′″ can be rotatedthereby providing a bayonet style locking mechanism of the disc member28′″ within the housing 12′″, as shown in FIG. 17.

A further alternative embodiment of the locking mechanism is shown inFIGS. 18 and 19. The housing member 12′″ includes a substantiallyarcuate recess 52″ having an open end portion 82 extending to an edge 84of the housing member 12′″. The recess 52″ includes a lip portion 86extending about a substantial portion thereof defining an inner groove88 between the seating surface 90 of the recess 52″ and the lip portion86. Arm portions 92, 94 are extensions of the lip portion 86 but extendfrom and are separate from peripheral ends 96, 98 of the housing member12′″ The arm portions 92, 94 have a spring-like quality such that theycan be deflected outwardly from the arcuate circle defined by the recess52″. Each of the arms 92, 94 has an elbow portion 100, 102 extendingfrom each arm portion 92, 94 towards the seating surface 90,respectively. The disc member 28′″ includes a substantially arcuateperipheral, radially outwardly extending flange portion 104. The flangeportion 104 includes two abutment edges 106, 108. In operation, theflange 104 and disc member 28′″ are disposed within the annular recessor groove 88, deflecting outwardly the arms 92, 94. Once disposed in therecess 52″, as shown in FIG. 19, the elbows 100, 102 engage the abutmentsurfaces 106, 108 of the disc member 28′″ thereby locking the discmember 28′″ in place. Outward deflection of the arms 92, 94 canselectively release the disc member 28′″ from locked engagement toprovide for further adjustment of the selection of the disc memberduring an operation procedure.

Also, as best shown in FIGS. 6 and 7, the pads members 31 can bedisposed in recesses 58, 60 in the lower and upper housing members 12′,14′ respectively. It is preferable to permanently adhere the pad members31 to the housing members 12′, 14′ by use of mechanical mechanismsand/or various adhesives, such as cyanoarylates, urethanes, and othermedical grade adhesives. This list of adhesives, as with other listingsof ingredients in the present application, is merely exemplary and notmeant to be exhaustive.

Examples of mechanical mechanisms for locking the pad members 31 intorecesses in the housing members are shown in FIGS. 20–23. One suchmechanism is an undercut locking mechanism shown in FIGS. 20–22. Housingmember 12′″″ includes a central recess 52 such as shown in FIG. 6 havinga ramp portion 56. The ramp portion 56 includes a centrally locatedtongue groove 57 allowing for the insertion of a spatula type deviceunder a disc member disposed within the recess 52 for releasing the discmember from the recess, similar to the use of a shoehorn type mechanism.Recesses 60′ include undercut recesses 110, 112 for locking engagementwith a peripheral flange portion 114 extending from an edge 116 of a padmember 31′. Since the pad member is made from a deflectable material,the flange portion 114 can be force-fit into and seated within theundercut 110, 112. The undercut locking mechanism effectively preventsthe pad member 31′ from disengagement with the housing member 12″″ insitu. Of course, the upper flange 118 would be locked within a similarundercut locking detail of recesses within the opposing housing member(not shown).

An alternative locking mechanism between the pad member and housingmember can be a tongue-and-groove relationship as shown in FIG. 23.Either the pad or the housing can include the tongue portion 122 and theother pad and housing members can include the groove 124. In otherwords, either of the locking members can include the tongue 122 and theother of the members being locked would include the groove 124. Analternative of this or the other locking mechanism shown is that therecess and/or pad can include multiple grooves or slots as well asmultiple tongues.

The various recesses or pockets 50′, 52, 58, 60 can be of differentrelative sizes and shapes. For example, the upper housing member 12′ mayhave a larger recess or pocket for seating a relatively larger one ofsaid discs 28 and the lower housing member 14′ may be include a smaller(larger and smaller referring to diameter of the annular recess) of therecesses or pockets for seating a relatively smaller one of the lowerdisc 30, thereby providing for an increased range of motion at thebearing surface interface.

The various Figures show that the outer surfaces 20, 22 of the variousembodiments of the housing members 12, 14 can include flanges generallyindicated at 60. The flanges 60 or fins, as they are sometimes referredto in the art, provide a mechanism for fixation to the intervertebralsurfaces. Various embodiments, such as those shown in FIGS. 1 and 2 aredual fin constructs. Other embodiments, such as those shown in FIGS. 8,12, and 13 are single fin or single flange constructs. Depending uponthe nature of the surfaces to which the outer surfaces 20, 22 are toabut, the surgeon can select various flange or fin configurations.Additionally, the fins 60 can be located in alternative positions,either centrally as shown in many of the Figures, or peripherally, asshown in FIG. 14, for a specific use with anterior extension plates, aswith screw fixations. The flanges, such as flange 60′″″″ can include abore 62 therethrough, which can be either a smooth surface or threadeddepending on its intended use.

The outer surfaces 20, 22 can be smooth, which allows for easierrevision as it allows for minimal to no ingrowth or they can betextured. Texturing of the outer surfaces 20, 22 allows ingrowth forlong-term fixation of the assembly 10. Porous coatings, plasma spray,grit blasting, machining, chemical etching, or milling are examples oftechniques for creating ingrowth capable surfaces. Coatings that enhancebone growth can also be applied. Examples of such coatings arehyroxyapatite and bone morphogenic proteins.

FIGS. 20 and 21 provide structure for further rotational stability ofthe device in situ. The housing member 12″″ includes pointed portions126, 128 extending from the outer surface 20′ thereof. The point members126, 128 function in conjunction with the flange portion 61′ to engagean opposing vertebral surface. The point portions 126, 128 beingdisposed radially peripherally from the centrally disposed flange 61′provide at least a three-point engagement of the vertebral surfacethereby preventing rotation of the housing member 12″″ relative thereto.Of course, the point portions 126, 128 can be in made in variousconfigurations and extend various amounts from the outer surface 20′ tobe custom suited to a specific vertebrae surface shape.

Alternatively, as shown in FIGS. 30–40, the disc 10″″″″ can be formed astwo separate pieces that are inserted into an intervertebral space,generally shown as 146 in FIG. 30. The benefit of this formation of thedisc 10″″″″ is that the discs 10″″″″ can be inserted during a posteriorinsertion. The two discs 10″″″″ function so that the units work intandem and effectively become one artificial disc assembly. Thearrangement of the two discs 10″″″″ enables each disc 10″″″″ to beinserted on either side of the spinal column into the intervertebralspace 146 and work in conjunction as a single artificial disc assembly10″″″″. The two discs 10″″″″ are angled toward the mid-line of thevertebral body 146. While two disc assemblies 10″″″″ are describedherein, more than two discs 10″″″″ can also be utilized withoutdeparting from the spirit of the present invention.

Each of the discs 10″″″″ include an upper housing member 12″″″″ and alower housing member 14″″″″. The housing members 12″″″″, 14″″″″ eachinclude a slot 35′ within the housing member 12″″″″, 14″″″″. The slot35′ enables the bearing 23 to move freely or “float” within the slot 35′in response to movement of the housing 14. As shown in FIGS. 31, 33–34,and 38–39, the slot 35′ can be formed in any shape that enables propermovement of the bearing 23, however, preferably the slot 35′ is anopen-ended u-shaped slot with a seat 39′ and side walls 37′. The sidewalls 37′ maintain the bearing 23 in proper alignment within the housing12″″″″, 14″″″″. As disclosed above, the bearing 23 is capable offloating within the slot 35′, thus enabling the bearing 23 to be mobileand free to move in any direction necessary to provide proper supportfor the housing 12″″″″, 14″″″″. The housing 12″″″″, 14″″″″ limits themotion of the bearing 23. The size of the housing 12″″″″, 14″″″″ and,more specifically, the slot 35′ in which the bearing 23 is disposedlimits the motion of the bearing 23. Further, bumpers 130, 132 can alsobe included in the slot 35′ to further limit the motion of the bearing23, provide dampening of the motion of the bearing 23 and prevent thebearing from being displaced from the housing 12″″″″, 14″″″″. Thebumpers 130, 132 can be of any size sufficient to provide the necessarylimitations on the bearing 23. For example, a single bumper can be usedfor both housings 12″″″″, 14″″″″. Alternatively, each housing 12″″″″,14″″″″ can incorporate separate bumpers 130,132. The bumpers 130,132 arealso useful for load sharing and thereby preventing the housing members12″″″″, 14″″″″ from contacting one another. The bumpers of the presentinvention 130, 132 are shaped to conform to the shape of the slot 35′.In other words, the bumpers 130, 132 are shaped to precisely fit theslot 35′ in which the bumpers 103, 132 are displaced. Preferably, thebumpers 130, 132 do not extend beyond the length of the housing 12″″″″,14″″″″. The bumpers 130, 132 have walls 134, 136 respectively thatengage the wall 37′ of the slot 35′. This enables the bumpers 130, 132to be maintained in alignment and prevents the bumpers 130, 132 frommoving.

The upper housing 12″″″″ can either include a slot 35′ identical to thatof the lower housing 14″″″″ or can include a single piece having amatching bearing that complements that of the bearing 23. In otherwords, the upper housing 12″″″″ can either have a slot 35′ that isidentical to the shape of the slot 35′ of the lower housing 14″″″″, suchthat the bearing 23 moves both in both housings 12″″″″, 14″″″″ equallyor the upper housing 12″″″″ can be formed such that only a single pieceis utilized and there is no movement within the top plate of the bearing23.

The bearing 23′ includes side arms 138, 140 that slidably engaged thewall 37′ of the slot 35′. The bearing 23′ is therefore held in positionwithin the slot 35′ via the side arms 138, 140 and the bumpers 130, 132.

The bearing 23 of the present invention can also have incorporated onthe bearing surface 24 various shapes as shown in the figures.Specifically, FIG. 32 shows the bearing surface 24′, wherein the surface24′ is a spherical surface. The spherical surface 24′ enables the centerof rotation of the bearing 23′ to exist at the center of the sphere.Therefore, the pair of discs 10″″″″ functions as a single artificialdisc with one center of rotation. Alternatively, the bearing 23 can havea surface that is either convex 24″ or concave 24′″. This embodiment isspecifically shown in FIGS. 9 and 10 wherein the center portion of thebearing 23′ is either convex or concave and there is a flat portion 29of the bearing 23′. When a convex or concave surface 24″, 24′″respectively, is utilized, the rotation center is not in the center forside-to-side rotation. Thus, the assembly is somewhat resistant toside-to-side bending but is more easily aligned.

The housings 12″″″″, 14″″″″ can be inserted simultaneously withoutincorporating the floating bearing 23 initially. This enables the disc10″″″″ to be inserted into the intervertebral space and once the disc10″″″″ has been inserted, the bumpers 130, 132 and the bearing 23 can beslid into place within the slot 35′. In another embodiment of thepresent invention, the lower housing member 12′″″″″ and the upperhousing member 14′″″″″ include a recess 52′″ for seating a positioningring 15, or spring mechanism 15, and bearing discs 28″″, 30″″ therein(See, FIGS. 41 and 42). Preferably, the recess 52′″ includes asubstantially arcuate peripheral undergroove 70″ or wall 70″ and abottom surface 19 that can be super finished smooth. The recess 52′″accommodates the positioning ring 15 therein and the undergroove 70″secures the positioning ring 15. The undergroove 70″ is defined by a lipportion 72″. The housings 12′″″″″″, 14′″″″″ include at least oneaperture 17 for insertion of screws therein and to secure the housings12′″″″″″, 14′″″″″ to a vertebral body. The positioning ring 15 can befixedly or removably attached to the housings 12′″″″″, 14′″″″″″Similarly, the bearing discs 28″″, 30″″ can be fixedly or removablyattached to the housings 12′″″″″, 14′″″″″.

The positioning ring 15, or spring member 15, is elastomeric and can bemade any material including, but not limited to, rubber, silicone,polyurethane, urethane composites, plastics, polymers, elastomers, andany other similar elastomeric material known to those of skill in theart. The positioning ring 15 is illustrated in detail in FIGS. 41–46.Preferably, the positioning ring 15 or spring member 15 is asubstantially annular body including an axially extended boretherethrough defining a passageway. Although the positioning ring iscircular in shape, any similar or appropriate design can be used such asan oval shape. Additionally, the substantially annular body has a seatextending radially inward towards the bore for seating therein thebearing discs 28, 30 and has an engaging member extending radiallyoutward from the bore for engaging the recess 52 of the housing member12, 14 and securing the positioning ring within the recess 52.Preferably, the engaging member can be any portion of the substantiallyannular body that radially extends from the bore. The engaging memberincludes, but is not limited to, a tapered edge, flange, and the like.The engaging member is shaped so as to be received by the recess and therecess securely engages the engaging member resulting in securing thepositioning ring within the recess.

The purpose of the positioning ring 15 or spring member 15 is to absorbcompressive loads between the bearing discs 28, 30 and the undergroove70″ or wall″ of the recess of the housing member, while controllingmotion and position of the bearing discs 28, 30. The positioning ring 15cushions and provides bias to absorb compression and lateral forces,while acting as a spring to re-center the bearing discs 28, 30 afterbeing displaced through vertebral function.

The bearing discs 28″″, 30″″ are situated within the opening of thepositioning ring 15 or spring mechanism 15. The bearing discs 28″″, 30″″can move within the positioning ring 15 and thus the housings 12′″″″″,14′″″″″ therein. However, movement within the housings 12′″″″″, 14′″″″″is semi-constrained by the positioning ring 15. The positioning ringacts as a spring to self-center the bearing discs 28″″, 30″″ and as ashock absorption member. As the bearing discs 28″″, 30″″ are free tofloat, the positioning ring 15 acts as a damper and self-centeringspring. Therefore, the bearing can translate in any direction, while thepositioning ring exerts a force to push the bearing back to center. Thefurther the bearing moves, the more force the positioning ring 15exerts. Any vertebral or spinal motion allows for load sharing anddamping of forces to the spine. As a load is transmitted, the bearingdiscs 28″″, 30″″ move and the force is shared by the positioning ring 15or spring mechanism 15.

In another embodiment of the present invention, the bearing discs 28′″″,30′″″ along with the positioning ring 15′ are oval shaped. Additionally,the recess 52″″ located on each housing member 12″″″″″, 14 ″″″″″ isoval-shaped, while the housing members 12, 14 can also be oval shaped,circular, or any other suitable shape known to those of skill in theart. The recess 52″″ accommodates the positioning ring 15′ therein andan undergroove 70′″ secures the positioning ring 15′. The undergroove70′″ is defined by a lip portion 72′″As shown in FIGS. 43-48, thebearing discs 28′″″, 30′″″ can be fixed within the oval recess 52′″ orthe bearing discs 28′″″, 30′″″ can be floating (i.e., mobile bearingdiscs) within the oval recess 52′″ of the housing members 12″″″″″,14″″″″″. The bearing discs 28′″″, 30′″″ have oval circumferentialexterior sides 21 and a spherical surface machined into the bearingsurface 24, 26. FIG. 44 illustrates the approximate shape of thepositioning ring 15′. FIG. 45 shows the positioning ring 15′ in placewithin the recess 52″″ and illustrates the oval shape in greater detail.FIG. 46 illustrates an upper housing member 14″″″″″, wherein the bearingdisc 30′″″ is fixed onto the upper housing member 14″″″″″. The exteriorcircumference of the bearing discs is oval, with the bearing surface 24,26 being spherical.

Under rotational loads, positioning ring 15′ engages the ovalcircumferential exterior sides 21 of the bearing discs 28′″″, 30′″″ andthe undergroove 70′″ of the recess 52″″ of the housing members 12″″″″″,14″″″″″. The greater the rotation, the more compressive force is exertedagainst the positioning ring 15′. Therefore, the disc 10 acts similar toa normal anatomic disc, whereby the annulus allows motion, but alsoprovides constraint of excessive motion. With such a rotation, thepositioning ring 15′ acts as a spring counteracting the rotationalforces to allow rotation, while preventing excess rotation therefrom.The positioning ring 15′ can be changed in durometer to create moremotion or less motion by altering the effective spring rate of thematerial. Thus, patient specific positioning rings 15′ can be chosenbased on patient requirements. In cases where facet joints aredeteriorated, the disc 10 can compensate by using a higher durometerpositioning ring 15′ and allowing the surgeon full optimization at thetime of surgery.

Under translation loads, the positioning ring 15′ acts as a spring toresist excessive motion, while acting as a spring to self-center thedisc construct. As shown in Figures, the oval aspect allows thenecessary engagement area to permit the combination of benefits. Also,by using such an oval surface, the positioning ring 15′ remains incompression at all times, allowing maximum benefit and performance fromvarious polymers. To one skilled in the art, the oval recess 52″″ couldbe any elongated surface that effectively provides some moment arm toexert force on the positioning ring 15′.

Various methods can be utilized for insertion of the present inventionin situ. For example, an assembled device 10 as shown in FIG. 1, can bedisposed between the intervertebral spaces during surgery, aftercalculation of space, depth, and height. Alternatively, opposing housingmembers 12, 14 can be disposed between the intervertebral spaces andpads 31 and disc members 24, 26 can be tested in situ prior to fixationthereof to allow for custom sizing. Accordingly, the present inventionbroadly provides a method of assembling an artificial intervertebraldisc 10 in vivo by inserting upper and lower housing members 12, 14 intoan intervertebral space and disposing cushioning pads 31 between theinner surfaces 16, 18 of the housing members 12, 14, thereby placing thepads in compression. The pair of disc members 28, 30 is inserted betweenthe inner surfaces of the plates 16, 18. The disc members 28, 30 haveabutting low friction surfaces 24, 26 therebetween. The disc members 28,30 are surrounded by the pads 31, whereby the disc members 28, 30 andpads 31 are under compressive forces and share such compressive forces.This step of the bearing surfaces 24, 26 and shock absorbing pads 31sharing absorption of the compressive forces and limiting the relativemovement of the housing members 12, 14 is an advantage not found in theprior art.

One use of the bearing of the present invention is in an artificialintervertebral disc for replacement of a damaged disc in the spine. Theartificial disc 10 of the present application includes a mobile bearing23 that allows for the bearing 23 to move to adjust and compensate forvertebral disc motion. By permitting the bearing to self-adjust, thebearing 23 can more freely move under translation loading conditionswhile maximizing the contact area of the upper and lower bearingsurfaces 20, 24.

In applications such as the lumbar spine, the disc upper member andlower member are angled relative to each other to maintain spinalcurvature. The load distributing damper and cushioning pads are alwaysunder some load when the spine is moving, although they can be adjustedfor a neutral no load situation when the spine is not moving.

The load distributing damper and cushioning pads also create an elasticmeans of self-centering the disc construct. Deflection of rotation ofthe disc forces the pads to act in such a way as to counter the force,thus allowing a unique self-centering capability. In an ideal situationwhere the patient's facets are uncompromised and ligamental balance isintact, this is not necessary. However, ligamental balance and damagedfacets would normally make an artificial disc questionable at best withthe current art. In such cases, having the ability to self-centeringcenter and restrict motion (the pads are elastic and thus restrictmotion by stretching and returning to rest), the possibilities ofextending indications to patients currently considered outside the scopeof artificial disc technology is highly advantageous. In a floatingbearing design, the ability to self-center mixed with the dampeningabilities of the pads creates an ideal system for an artificial disc.

The pads can also be adjusted according to patient and surgeonrequirements. In such cases where range of motion needs to be restricteddue to compromised facets, a harder, less elastic pad can be inserted.Since a less elastic pad moves and stretches less, the disc isautomatically restricted in motion. This method of adjusting pads can bedone interoperatively to compensate for surgical and patient conditions.

As described above, any of the above embodiments can be used in acervical disc surgical procedure. With regard to the embodiment of thehousing members 12, 14 illustrated in FIGS. 41 and 42, the generalprocedure begins with the removal of the damaged disc (FIGS. 49–57illustrate the procedure). Then, a trial handle is attached to the trialand the trial is inserted into the disc space (FIG. 49). The trial isadjusted until the disc height is approximately restored, while beingcareful not to overstretch the ligaments. Using a drill guide, pilotholes are drilled at the four guide plate hole locations (FIG. 50). Theguide plate is secured with self-tapping guide plate screws (FIG. 51).Using the end plate preparation instrument, reaming disks are insertedto match the trial number. The depth of the instrument on the dial tothe matching number must then be set. Once set, the instrument isadvanced into the disc space with the button engaged (FIG. 52). The finson the instrument remain engaged in the slot on the guide plate forstability. Once maximum depth is reached, the end plate preparationinstrument is removed (FIG. 53). The guide plate screws and guide plateare then removed (FIG. 54). The disc holder with holes in plate alignedwith holes in the vertebrae is inserted until fully seated (FIG. 55).Screws are then inserted into threaded holes to secure disc to thevertebral bodies (FIG. 56). Finally, the disc inserter is removed (FIG.57).

Throughout this application, various publications, including UnitedStates patents, are referenced by author and year and patents by number.Full citations for the publications are listed below. The disclosures ofthese publications and patents in their entireties are herebyincorporated by reference into this application in order to more fullydescribe the state of the art to which this invention pertains.

The invention has been described in an illustrative manner, and it is tobe understood that the terminology that has been used is intended to bein the nature of words of description rather than of limitation.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is, therefore, to beunderstood that within the scope of the appended claims, the inventioncan be practiced otherwise than as specifically described.

1. An artificial intervertebral disc comprising: housing members havingspaced inner surfaces, at least one inner surface having a recess and aperipheral groove extending outwardly from the recess; a bearingoperatively disposed between the inner surfaces for moving relative tothe housing members to adjust and compensate for vertebral disc motion,the bearing comprising a first bearing disc and a second bearing discslidably engaging the first bearing disc along a bearing surface; and aresilient flexible positioning ring surrounding the bearing forabsorbing shock and centering the bearing in response to movement, thepositioning ring having a body portion and an engaging member extendingradially outwardly from the body portion into the peripheral groove tosecure the positioning ring in at least one of the housing members. 2.The disc according to claim 1, wherein the positioning ring forms anaxial bore, and the first and second bearing discs engage one anotherwithin the axial bore.
 3. The disc according to claim 1, wherein theengaging member includes a generally circular flange extending outwardlyfrom the perimeter of the positioning ring.
 4. The disc according toclaim 3, wherein the peripheral groove is generally circular, and theradius of the flange and radius of the groove are substantiallyidentical.
 5. The disc according to claim 1 wherein the housing membersinclude an outer surface having a coating thereon selected from thegroup consisting essentially of TiN (Titanium Nitride), diamond,diamond-like materials, synthetic carbon-based materials, andchromium-based materials.
 6. The disc according to claim 1, wherein thebearing is constructed from a composition selected from the groupconsisting essentially of metals, ceramics, and plastics.
 7. The discaccording to claim 1, wherein the positioning ring is made of materialsselected from the group consisting essentially of rubber, silicone,polyurethane, urethane composites, plastics, polymers and elastomers. 8.The disc according to claim 1, wherein the housing members include atleast one aperture for accommodating at least one bone screw.
 9. Anartificial intervertebral disc comprising: housing members having spacedinner surfaces, at least one inner surface having a recess and anannular lip extending radially inwardly above the recess; a bearingoperatively disposed between the inner surfaces for moving relative tothe housing members to adjust and compensate for vertebral disc motion,the bearing comprising a first bearing disc and a second bearing discslidably engaging the first bearing disc along a bearing surface; and aresilient flexible positioning ring surrounding the bearing forabsorbing shock and centering the bearing in response to movement, thepositioning ring having a body portion and an engaging member extendingradially outwardly from the body portion into engagement with theannular lip to fix the axial position of the positioning ring relativeto at least one of the housing members.
 10. The disc according to claim9, wherein the positioning ring forms an axial bore, and the first andsecond bearing discs engage one another within the axial bore.
 11. Thedisc according to claim 9, wherein the engaging member includes agenerally circular flange extending outwardly from the perimeter of thepositioning ring.
 12. The disc according to claim 11, wherein theperipheral groove is generally circular, and the radius of curvature ofthe flange and groove are substantially identical.
 13. The discaccording to claim 12, wherein the housing members include an outersurface having a coating thereon selected from the group consistingessentially of TiN (Titanium Nitride), diamond, diamond-like materials,synthetic carbon-based materials, and chromium-based materials.
 14. Thedisc according to claim 9, wherein the bearing is constructed from acomposition selected from the group consisting essentially of metals,ceramics, and plastics.
 15. The disc according to claim 9, wherein thepositioning ring is made of materials selected from the group consistingessentially of rubber, silicone, polyurethane, urethane composites,plastics, polymers and elastomers.
 16. The disc according to claim 9,wherein the housing members include at least one aperture foraccommodating at least one bone screw.